The High P-T Stability of Hydroxyl-apatite in Natural and Simplified MORB-an Experimental Study to 15 GPa with Implications for Transport and Storage of Phosphorus and Halogens in Subduction Zones

Experiments have been conducted in the range 3-15 GPa and 850-1800 degrees C to investigate the P-T stability field of OH-apatite in an average mid-ocean ridge basalt (MORB) and a model Mg-basalt, to study the compositional evolution of apatite and its breakdown products and the partitioning of P between phosphates and silicates. In the bulk compositions investigated OH-apatite is stable to < 7 center dot 5 GPa at 950 degrees C in a typical eclogite assemblage garnet + omphacite + SiO2 + TiO2. This is similar to 5 GPa below the breakdown P of pure OH-apatite. The high-P breakdown product is tuite [gamma-Ca-3(PO4)(2)]. Both apatite and tuite are stable in a wide range of subduction zone T regimes but not along an average mantle adiabat. This precludes apatite or tuite stability in the asthenospheric mantle. Apatite may be stable in cold continental lithosphere (40 mW/m(2)) but is restricted to P < similar to 4-5 GPa. The apatite breakdown reaction is an important limit for the crust-mantle transport of Cl in subduction zones and can contribute to the Cl depletion of subducted cust. Both apatite and tuite are important storage sites for large ion lithophile elements (LILE) and rare earth elements (REE), therefore apatite breakdown does not greatly affect LILE or REE transport in subduction zones. In an eclogite assemblage only garnet can accommodate significant P. In the presence of apatite or tuite, P2O5 contents in garnet range from similar to 0 center dot 2 to 0 center dot 6 wt % between 3 and 11 GPa and increase to similar to 0 center dot 8 wt % at 15 GPa in the absence of a detectable phosphate phase. The P-storage capacity of clinopyroxene is limited to similar to 250 ppm. Because of the extreme preference of P for the garnet structure, virtually the entire P budget of subducted MORB will be locked up in garnet well into the lower mantle provided fO(2) is high enough to prevent the stability of a metal phase.